Hearing aid apparatus



19387 w. D. PENN 2,140,969 I HEARING AID APPARATUS l 2 Sheets-Sheet 1 Filed June 19, 1937 ar-l.

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ATT EY Dec. 20, 1938. w PENN 2,140,969

\ HEARING AID APPARATUS Filed June 19, 1957 2 Sheets-Sheet 2 I INVENTOR. %ZZZ027L 2 P92222 ATTO EY Patented Dec. 20, 1938' UNITED STATES.

PATENT OFFICE HEARING AID APPARATUS William D. Penn, Dallas, Tex. Application June 19, 1937, Serial No. 149,271

8 Claims.

The invention relates to a method and appsratus for aiding hearing and more particularly to an apparatus and method for selectively accentuating the frequencies to which a particular user is less sensitive than normal.

This application is a continuation in part of my application Serial No. 39,073 filed September 3, 1935.

The invention is particularly adapted for use in connection with individuals affected with nerve deafness, for in such cases the hearing loss is small at the lower frequencies and much greater at the higher frequencies. Unless the hearing device is designed to amplify selectively, accentuating the higher frequencies to a greater extent than the lower. the lower frequencies to which the individual is nearly as sensitive as a normal individual will be amplified to the threshold of feeling, causing considerable discomfort to the user aswell as rendering the sound unintelligible.

An object of the invention is, therefore, to provide a device and method of selectively amplifying the frequencies to which a particular person is least sensitive.

Another object of the invention is to provide a device, having no more elements than conventional hearing aid devices, which will amplify higher frequencies within the audible range to a greater extent than lower frequencies.

Hearing devices employing vacuum tube am:

pliiiers or other types having one or more transformers in circuit can be made to accentuate certain frequencies by providing certain relationships between the transformers and the impedances connected to them. It is therefore a further object of the invention to. provide a novel device and method in which a transformer of predetermined characteristics is employed to selectively amplify the frequencies desired.

Other objects and advantages of the invention will appear in the following detailed description of the preferred embodiments of the invention shown in the attached drawings of which Fig, l is a schematic diagram of one embodiment of the invention using a single transformer; and

Fig. 2 is a schematic diagram of a modification thereof in which two transformers are employed.

Fig. 3 is a schematic diagram of a further modification of this invention, showing an adjustable type transformer.

Figs. 4, 5 and 6 are schematic diagrams employed for the purpose of illustrating and facilitating the explanation of features of this invention.

Referring more particularly to the drawings,

l0 indicates a microphone, ii indicates a coupling transformer between the microphone and amplifier tube l2 and It indicates a telephone receiver unit or bone conduction unit. The microphone I0 is connected to the primary ll of the transformer II, in series with a suitable source of voltage shown as a microphone battery it. The secondary ill of the transformer II has a suitable high resistance I! connected there across with a variable contact it to provide a volume control. Suitable sources of potential, shown as batteries l9 and 20, are provided for the filament, grid and plate circuits respectively of the tube II.

As has been 'hereinbefore indicated, the circuit shown in the drawings can be made to selectively amplify certain frequencies by a proper relation between transformer and terminating impedances. The transformer I prefer to employ is provided with windings of very low distributed capacity and the leakage inductance referred to the primary winding is preferably less than one millihenry. These conditions may be obtained by making the physical dimensions of the transformer extremely small. For example, the transformer may be made approximately 1.2 inches wide, 1.1 inches high and 1.0 inch overall thickness. In any event, the only necessary restriction is that the leakage inductance referred to the primary winding and the distributed capacity (including the capacity of the associated circuits connected to the transformer) referred to the primary, be of 'such a magnitude compared to the impedance connected to the primary winding such that they do not appreciably affect the circuit at the lower audio frequencies below the first resonant frequency of the transformer. The first resonant frequency is that due-to resonance between the primary inductance and the effective capacity referred to the primary.

Insofar as the action of the transformer itself is concerned it may be shown that in the range ill below the flrstresonant frequency the voltage In these equations:

es=the voltage across the load circuit at a given frequency. 6o=th8 open circuit output voltage of the microphone' I! at the same frequency.- Ro=the internal impedance of the microphone. wL1=the open circuit reactance of the primary ll of the transformer H. w=21rf.

. R =the impedance of the control I1 referred to the primary =(m/ni) times resistance of H.

k=the co-eiiicient of coupling of the transformer.

The vacuum tube and its load may easily be adjusted to give a substantially constant amplification over the entire audio frequency range under consideration and hence the voltage ratio given by the above equation is in effect merely multiplied by a constant factor due to the vacuum tube.

From Equation 1 it is seen that in the range for which the equation applies the voltage ratio depends, for any given value of R0 and "2/ 1 upon the ratio Ro/Liwk For a closely coupled transformer k is approximately equal to unity.

By choosing the proper value of Ra/L'mflc: the voltage amplification can be caused to fall off as the frequency decreases.

Equation (2) gives the decibel loss due to the shunting effect of the primary. If the ratio Ro/Liwk is sumciently small Equation (2) would become 1/( o/R)' inim and there would be no loss due to the-shunting effect.

If the secondary of the transformer Ii is assumed to work open circuited then Rl= and and y-4+ (ISO/hwy 1 hence (Ra/L110) :398, w=21rf=377 so 200 L1-3 77XTT9-0.0266=26.6 m. h.

Thus for the assumed conditions we have obtained from Equation 2 the value of L1 necessary to attenuate the c. p. s. frequency 20 decibels below the normal'sain of thedevice.

It is understood, of course, that the transformer may be placed in other positions than that shown in Fig. 1, without departing from the spirit of my invention. For example, as shown in Fig. 2, the receiving unit I! is coupled to the output of the tube II by a transformer 22 having a primary 28 in the plate circuit of the tube Ii, and a secondary 24 connected to the tube lIa the output of which is connected to the receiving unit II. This second transformer may be employed to selectively amplify the desired frequencies. In

this case, the ratio determining the frequency response characteristic of the transformer 12 is r /Luv instead of Ro/L1w as in Equations '1 and 2, where r, is the of the tube variational plate resistance :2 and 1 1.; is the open circuit reactance of the primary 2!.

transformers of Fig. 2, the range of control may be advantageously extended.

In the case of the transformer 22, in the plate circuit of the vacuum tube, the equivalent a. c. voltage effective in the plate circuit acts in series through the variational or dynamic plate resistance, Tp of the tube.

The voltage acting in the plate circuit is we, where u is the amplification factor of the tube l2 and 8 the voltage at the grid of the tube. In other words, the vacuum tube can be replaced by a voltage u acting in series with the. plate resistance r, of the tube, as far as external performance is concerned. This is represented schematically in Fig. 4.

Equation (1) then becomes:

1 r,' R (r /L 1W3 For any particular stage then of the amplifier, s=voltage across the load circuit, e,=the voltage applied to the grid of the tube. This equation applied to the transformer coupled stage of the amplifier indicated between the brokenlines A-A and 3-13 of Fig. 5. This portion of the circuit of Fig. 5 is equivalent to the circuit shown in Fig. 6. This equation is the same as Equation 1) except that the dynamic plate resistance of the vacuum tube has been substituted for the microphone resistance, e; has been substituted for o and the right hand member of the equation has been multiplied by the factor u to take account of the gain of the vacuum ,tube.

Where the ratio of the turns of the transformer secondary It to the turns of the primary H is such that the equivalent load resistance referred to the primary is large in comparison to the impedance of the microphone 10 so as to make the ratio of the impedance of said microphone to the equivalent load resistance referred to the primary negligible in comparison to unity, Equation 1 resolves itself into the following equation:

The above relation shows that the amplification depends upon the ratio TL, which is the ratio of the microphone impedance to the reactance of the transformer primary. Since the smallest value which the expression under the radical can have in the above equation is unity, the factor A also represents the maximum value of amplification possible. The value of the expression wLi increases with frequency which indicates that the amplification increases with frequency. For example, at the frequency which makes Ru=wL1, .the amplification will be 70.7%

of the maximum value and at three times this frequency the amplification: will be of this maximum. Thus, by selecting a proper ratio of & I the amplification at the low frequencies can be made a small fraction of the maximum available amplification and as the frequency increases the amplification will also increase until the maximum value is reached.

Also by utilizing both As a specific example of the application of the a .\/1+(WL1 when the frequency is 60 c. p. s. is made equal to 5,. This requires that the ratio If the microphone impedance in the particular circuit is, for example, 200 ohms, then 0.1 of the maximum at 60 c. p. s. 0.64 of the maximum at 500 c. p. s. 0.81 of the maximum at 1000 c. p. s. 0.95 of the maximum at 4000 c. p. s.

This is a very close approach to the ideal characteristic desired and no further calculations are necessary.

In Fig. 3 I have shown another modification of a device having a transformer H with an adjustable frequencyresponse characteristic. In this modification the microphone i0 connected in series with a battery I! isconnected to an alterable tap primary H of the transformer Ii. By means of the taps 30 the effective inductance of the primary may be altered and hence the frequency response characteristic changed. As has been shown elsewhere in the specification. with the other elements fixed in the systems disclosed in Fig. 3, the amplification obtainable depends primarily upon the ratio of the impedance of the microphone ill to the reactance of the primary ll of the transformer. This ratio willvary with different frequencies and by changing the connection of the microphone ill with the taps 30, the variation of this ratio can be changed to cause the system to amplify selectively the frequencies to which the particular deaf person being aided is least sensitive. Also the various taps 30 of the primary il may be interconnected in various ways to change the frequency response characteristic of the transformer. for various impedances may be connected across the taps either singly or in combination to vary the selective amplification of the system. It is pointed out that if the deafness is slight, the amplifying tubes may be omitted and the adjustable frequency responsive transformer used to couple the microphone ill to the receiving device ll.

WhileI have disclosed the preferred embodiments of my invention and have given specific examples thereof, it is understood that I am not to be limited thereto. but that the invention may be varied within the scope of the following claims:

I claim: 1

1. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and am-' plify the high audible frequencies comprising: a microphone pick-up adapted to translate mechanical vibrations into corresponding variable electrical currents, said microphone pick-up having a predetermined impedance characteristic, a transformer having primary and secondary windings, connections between said microphone pick-up and said primary, and a load circult including a reproducing device connected to said secondary. the open circuit impedance of the primary and the impedance of said microphone being such that the voltage ratio es/eo of the said apparatus is equal to wherein es is the voltage across the load circuit,

e0 is the open circuit voltage of the microphone, m/ni is the turn ratio of the transformer, Rois the microphone impedance, R is the effective load impedance referred to the primary, k is the coefilcient of coupling of the transformer, and win is the open circuit reactance of the primary. whereby the low audible frequencies are attenuated selectively to the high audible frequencies.

2. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and amplify the high audible frequencies comprising: a microphone pick-up adapted to translate mechanical vibrations into corresponding variable electrical currents, said microphone pick-up having a predetermined impedance characteristic. a transformer having primary and secondary windings, connections between said microphone pick-up and said primary, and a load circuit including a reproducing device connected to said secondary, the ratio of the turns of said second-' ary to the turns of said primary being such that the equivalent load resistance referred to the primary is large in comparison to the impedance of said microphone so as to make the ratio of the impedance of said microphone to the equivalent load resistance referred to the primary negligible in comparison with unity. the open circuit impedance of the primary and the impedance. of

said microphone being such that the voltage ratio es/eo of the said apparatus is equal to Rn x zsni wherein es is the voltage across the load circuit. 60 is the open circuit voltage of the microphone,

A is a circuit constant. R0 is the mean microphone impedance and wLi is the effective reactance of the primary, whereby the low audible frequencies are attenuated selectively to the high audible frequencies.

3. Hearing aid apparatus adapted selectively to attenuate the low audible frequenc es and amplify the high audible frequencies comprising: a microphone pick-up adapted to translate mechanical vibrations into corresponding variable electrical currents, said microphone pick-up having a predetermined impedance characteristic. a transformer having primary and secondary windings, connections between said microphone pick-up and said primary, and a load circuit including a reproducing device connected to said secondary. the open circuit impedance of the primary and the impedance of said microphone being such that the voltage ratio es/eo of the said apparatus is equal to ("z/ a 4(1 Ro/RP-i- (Rn/L Vk) wherein es is the voltage across the load circuit. an is the open circuit voltage of the microphone, m/ni is the turn ratio .of the transformer, Re is the microphone impedance, R is the effective load impedance referred to the primary, is is the co-efiicient of coupling of the transformer, and wL1 is the open circuit reactance of the primary, the ratio Ro/Liwkfi in the aforesaid equation being of such a value that the voltage amplification eb/eo is caused to fall off as the frequency decreases, whereby the low audible frequencies are attenuated selectively to the high audible frequencies.

4. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and amplify the high audible frequencies comprising: a microphone pick-up adapted to translate mechanical vibrations into corresponding variable electrical currents, said microphone pick-up having a predetermined impedance characteristic, -a transformer having primary and secondary windings, connections between said microphone pick-up and said primary, a vacuum tube amplifier connected to said secondary winding, and a load circuit including a reproducing device connected. to said vacuum tube amplifier, the open circuit impedance of the primary and the impedance of said microphone being such that the voltage ratio et/eo of the said apparatus is equal to m/m is the turn ratio of the transformer, R is 1 the microphone impedance, R is the effective load impedance referred to the primary, A isa circuit constant including the amplification of the vacuum tube amplifier, k is the coefficient of coupling of the transformer, and wLl is the open circuit reactance of the primary, whereby the low audible frequencies are attenuated selectively to the high audible frequencies.

5. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and amplify the high audible frequencies comprising: a micro phone pick-up adapted to translate mechanical vibrations into corresponding variable electrical currents, said microphone pick-up having a predetermined impedance characteristic, a transformer having primary and secondary windings, connections between said microphone pick-up and "said primary, a vacuum tube amplifier con nected to said secondary, and a load circuit including a reproducing device connected to said vacuum tube amplifier, the open circuit impedance of the primary and the impedance of said microphone being such that the voltage ratio I et/eo of the said apparatus is equal to a/ 1 1/ 1+Ro/R =+(Ro/ 1wk*)' wherein es is the voltage across the load circuit,

60 is the open circuit voltage of the microphone,

112/111 is the turn ratio of the transformer, R0 is the microphone impedance, R is the effective load impedance referred to the primary, A is a circuit constant including the amplification of the vacuum tube amplifier, k is the coefilcient of coupling of the transformer, and wLi is the open circuit reactance of the primary, the ratio Ru/Liwk in the aforesaid equation being of such a value that the voltage amplification Cb/CO is caused to fall off as the frequency decreases, whereby the low audible frequencies are attenuated selectively to the high audible frequencies.

6. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and amtransformer and the dynamic plate resistance of the vacuum tube connected to said primary being such that the voltage ratio eb/e of said transformer coupled stage is equal to w/( 1' wherein en is the voltage across the load circuit connected to the secondary of said transformer, c, is the voltage applied to the grid of saidvac uum tube, 11. is the amplification factor or variational plate voltage ratio nz/m is the turn ratio of the transformer, r, is the dynamic plate resistance, R is the efiective load impedance referred to the primary, is is the co-efilcient of coupling of the transformer, and Luv is the open circuit reactance of the primary, whereby the low audible frequencies are attenuated selectively to the highaudible frequencies.

7. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and amplify the high audible frequencies comprising: a

pick-up device adapted to translate mechanical vibrations into corresponding variable electrical currents} an amplifier therefor including at least one vacuum tube and transformer coupled stage of amplification, a transformer and a vacuum tube for said amplifier, an output circuit including a reproducing device connected to said amplifier, the open circuit reactance of the primary of said transformer and the dynamic plate resistance of the vacuum tube connected to the primary of said transformer being such that the voltage ratio et/eq of said transformer coupled stage is equal to connected to the secondary of said transformer, e, is the voltage applied to the grid of saidvacuum tube, u is the amplification factor or variational plate voltage ratio of said vacuum tube, nz/m is the turn ratio of the transformer, r,

is the dynamic plate resistance R is the effective load impedance referred to the primary, is

is the co-eflicient of coupling of the transformer, and 101.1 is the open circuit reactance of the primary, the ratio 1- /L1wk in the aforesaid equation being of such a value that the voltage amplification en/ev is caused to fall off as the frequency decreases, whereby the low audible frequencies are attenuated selectively the the high audible frequencies. I

8. Hearing aid apparatus adapted selectively to attenuate the low audible frequencies and amplify the high audible frequencies comprising: a pick-up adapted to translate mechanical vibrations into corresr nding variable electrical currents, said pick-up having a predetermined impedance characteristic, a transformer having primary and secondary windings, connections be- 'to said secondary winding, a second transformer having a primary connected to the plate circuit f- 1/0+eI +(rt/Lan wherein e1, is the voltage across the load circuit connected to the secondary of said second transformer, e, is the voltage applied to the grid of said vacuum tube, u is the amplification factor or variational plate voltage ratio of said vacuum tube, m/m is the turn ratio of the transformer, r, is the dynamic plate resistance, R is the effective load impedance referred to the primary of said second transformer, k is the co-eflicient of coupling of said second transformer, and wLi is the open circuit reactance of the primary of said 10 second transformer, whereby the low audible frequencies are attenuated selectively to the high audible frequencies. WILLIAM D. PENN.

CERTIFICATE OF CORRECTION.

Patent No. 2,1il0,969.

December 20, 1958 WILLIAM D. PENN. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correctionas follows:

"decimels" read decibels; and second column, line 214,

column, line L l, for

Page 2, first for "applied" read applies; and that the said Letters Patent shouldbe read with this correction therein that the same may conform to the record of the case in the Patent Office.

signed and sealed this 5rd day of October, A. D. 1959.

(Seal) Henry Van Arsdale, Acting Commissioner of Patents. 

